1. Field of the Invention
The present invention relates to an improved pultrusion apparatus and method of use thereof for continuous processing or working of a material to provide a shaped cross-section. In particular the present invention relates to a pultrusion apparatus which is adapted to be modified for the processing of materials with different cross-sections without reconstruction of the microwave portion of the apparatus.
2. Description of Related Art
The pultrusion process is a continuous manufacturing method used to produce fiber reinforced plastic profiles with a constant cross-sectional area through the length of the product. In pultrusion, the product's shape is determined by continuously pulling the composite material through a die. Pultruded composites consist of reinforcing materials, a resin that binds the fibers together, and often a mat material to improve the appearance of the composite's surface and other ancillary materials (R. W. Meyer, Handbook of Pultrusion Technology, Chapman and Hall, New York, pp. 1 to 7 (1985)). The key step in a pultrusion process is to control the interactions among fiber, resin and additives. The conventional processing method is to heat the materials in the die through thermal convection or conduction. Either of the thermal heating methods conveys heat through the surface of the material. These heating methods often cause uneven heating in large parts.
Compared to the thermal heating method, microwave curing of composite materials is a faster and more direct heating method that takes advantage of the dielectric properties of the material being processed. The general advantages of using microwave technology include: shorter processing time, better control of temperature profiles within the composite material, and improved mechanical properties of the final part (J. Wei, et al., SAMPE J., 27 (1), 33, (1991)). Microwave heating devices such as commercial microwave ovens (W. I. Lee and G. S. Springer, J. Comp. Mater., 18, 387 (1984)) and waveguides (W. I. Lee and G. S. Springer, J. Comp. Mater., 18, 357 (1984)) have also been studied. A continuous microwave processing technique is highly desired for fast processing of large composite parts, such as long pipes or panels. The microwave leakage during continuous processing of non-conductive materials has been controlled by attaching extended entry and exit ports to the microwave cavity (A. L. VanKoughnett and J. G. Dunn, J. Microwave Power, 8 (1), 101 (1973)).
The applications of continuous microwave technique in industries has been under investigation for decades. Most of these applications are using microwave energy to preheat or to postcure materials after parts being made (S. H. Munson-McGee, Opportunities for Innovation: Polymer Composites, NIST GCR 90-577-1, pp. 121 (August, 1990)). One pultrusion application of processing materials continuously in a waveguide has been patented (UK Pat. 2,245,893, A. Cooper and J. M. Methven (1990)). The idea of using a waveguide as part of a pultrusion die is excellent in terms of the simplicity of the system. However, the dimension of the waveguide has to accord with the dielectric properties of the materials being processed in order to create the suitable microwave field patterns (S. R. Ghaffariyan and J. M. Methven, Mat. Res. Soc. Symp. Proc., 189, 135 (1991)). This requirement has greatly limited the wide application of the technique in industry due to the large variety of materials being processed.
U.S. Pat. No. 4,792,772 to Asmussen describes a general purpose microwave applicator which is commercially available. The unique feature of this apparatus is that it allows for precise tuning for mode selection and fine tuning within a mode selected. Other related prior art patents are U.S. Pat. Nos. 4,440,593, 4,507,588, 4,585,668, 4,777,336, 4,727,293, 4,630,566, 4,691,662, 4,943,345, 4,906,900, 5,081,398, and 5,008,506. This apparatus is preferred for the present invention.
The problem which was not solved in using this apparatus is how to seal the cavity when the material is being continuously processed through the microwave cavity.
U.S. Pat. Nos. 3,793,108 to Goldsworthy, 3,966,533 to Goldsworthy et al, 4,151,031 to Goad et al, 4,469,541 to Goldsworthy, 4,477,707 to Kim, British Patent Application No. 2,245,893 to Cooper et al and French Patent Application No. 2,397,281 assigned to SAEI Celite describe various methods for pultrusion using microwave energy curing. In all of these devices the microwaves are assumed to be generated without means for adjusting either the mode or fine tuning the mode of the microwaves.
The problem not solved by the prior art is how to provide a microwave apparatus which is adaptable to many different applications of continuous pultrusion without building a special purpose apparatus for each application and which is safe such that the microwaves are sealed in the cavity.